Molybdenum (Mo) that has, for example, a high melting point (about 2600° C.), relatively high mechanical strength superior to other metals having high melting points, a low thermal expansion coefficient, excellent electrical conduction and thermal conduction properties, and a high corrosion resistance to a melted alkali metal and hydrochloric acid, can be applied to, for example, electrodes, components for vessels, components for semiconductors, components for heat-resistant structures, and materials for nuclear reactors.
A worked material having a worked structure exhibits high toughness due to suppressed crack growth. However, in a material recrystallized by heating (about 1050° C. or more), strength at high temperatures is not satisfactory because a crack readily grows to cause embrittlement. Therefore, Mo—Ti(0.5)—Zr(0.08)—C(0.03) (TZM) alloy and Mo—Nb(1.5)—Ti(0.5)—Zr(0.03)—C(0.03) (TZC) alloy have been developed as molybdenum alloys having improved strength at high temperatures.
The inventors found that, in a worked refractory-metal-alloy material such as an ultrafine-nitride-containing molybdenum alloy formed by multi-step internal nitriding treatment, high toughness and high strength are achieved by maintaining a worked structure in at least the surface region of the worked material (patent document 1, non-patent documents 1 to 3).    Patent document 1: Japanese Unexamined Patent Application Publication No. 2001-73060.    Non-patent document 1: Masahiro Nagae, Jun Takada, Yoshito Takemoto, Yutaka Hiraoka, and Tetsuo Yoshio. J. Japan Inst. Metals, 64(2000)747-750.    Non-patent document 2: Masahiro Nagae, Jun Takada, Yutaka Hiraoka, Yoshito Takemoto, and Tetsuo Yoshio. J. Japan Inst. Metals, 64(2000)751-754.    Non-patent document 3: Masahiro Nagae, Jun Takada, Yutaka Hiraoka, and Yoshito Takemoto, Materia Japan, 40(2001)666-667.